Catch backdrop exceptions and display in admin UI

* Change approach of using response_add and response_change to
changelist_view which covers all scenarios
* New approach is to delete the success message from the django
messages queue rather than intercepting it. Because django 1.6 does
not allow intercpeting the delete success message. django 1.7 does
support this however if we upgrade. http://stackoverflow.com/a/18289950
* Override log_deletion and delete_model as per existing add and
change
* Factor out backdrop exception handler to support both save_model and
delete_model